Meiotic recombination is required to assure proper disjunction of homologous chromosomes at anaphase I. Recombination can be studied genetically by linkage analysis, or cytologically by analysis of chiasmata at diakinesis/metaphase I. Genetic linkage maps provide information on gene order and crossover frequency but not on the influence of chromosome structure on recombination, while chiasmata analysis provides global counts, but no information on frequency of recombination of any two linked genes. To bridge this gap, we have developed a new technique that combines fluorescent antibody localization of MLH1, a mismatch repair protein that marks crossover sites of meiotic prophase chromosomes (synaptonemal complex or SC) spreads with multicolor FISH (mFISH) of whole chromosome libraries to identify all chromosomes in the meiotic prophase nucleus and map sites of recombination along their lengths. In addition we can now re-probe the nuclei with differentially labeled BACs of known genetic or cytogenetic "addresses". This will allow us to integrate the MLH1 maps and the linkage maps. We propose to use this technique to construct physical recombination maps from human spermatocytes and continue to build more complete maps for the mouse. Combining the two maps will allow us to identify factors that may influence crossover frequency and distribution. Factors likely to be important include chromosome length, heterochromatin, R-bands versus G-bands, centromeres, telomeres, satellite sequences and insulators (sequences and associated BORIS protein). Once we identify potential control factors, we will assess their relative importance by examining changes in recombination frequency and distribution in selected chromosome aberrations in the mouse. In addition, we will examine the effects of mutations in genes likely to be involved in control of frequency and distribution of recombination.